Modeling the natural attenuation of benzene in groundwater impacted by ethanol‐blended fuels: Effect of ethanol content on the lifespan and maximum length of benzene plumes

Abstract

A numerical model was used to evaluate how the concentration of ethanol in reformulated gasoline affects the length and longevity of benzene plumes in fuel‐contaminated groundwater. Simulations considered a decaying light nonaqueous phase liquid source with a total mass of ∼85 kg and a groundwater seepage velocity of 9 cm d−1 and corroborated previous laboratory, field, and modeling studies showing benzene plume elongation due to the presence of ethanol. Benzene plume elongation reached a maximum of 59% for 20% ethanol content (E20) relative to regular gasoline without ethanol. Elongation was due to accelerated depletion of dissolved oxygen during ethanol degradation and to a lower specific rate of benzene utilization caused by metabolic flux dilution and catabolite repression. The lifespan of benzene plumes was shorter for all ethanol blends compared to regular gasoline (e.g., 17 years for regular gasoline, 15 years for E10, 9 years for E50, and 3 years for E85), indicating greater natural attenuation potential for higher‐ethanol blends. This was attributed to a lower mass of benzene released for higher‐ethanol blends and increased microbial activity associated with fortuitous growth of benzene degraders on ethanol. Whereas site‐specific conditions will determine actual benzene plume length and longevity, these decaying‐source simulations imply that high‐ethanol blends (e.g., E85) pose a lower risk of benzene reaching a receptor via groundwater migration than low‐ethanol blends such as E10.